Here are the basics, I think.

Two reasons.

#1
Looking from behind:
The prop spins clockwise so conservation of momentum would suggest the plane wants to spin counter-clockwise. Thus, the left wing tips down and the plane wants to turn left.

#2
The air coming off the propeller is swirling. At the rudder the air flow comes from the left. This give a lift vector at the rudder that pushes the rudder right and also make the plane turn left.

Both of these effects change with throttle setting. So the rudder needs to be adjusted with throttle to make the plane fly straight.

If you trim the surfaces to fly straight at low throttle and then trim the right thrust to fly straight at full throttle, you will have a plane that doesn't change heading much with throttle setting.

If you do search on trimming or trimming charts you can look-up the best ways to check the right and down trust.

Carl